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flow: Fix incorrect hash probe in flowside_lookup()

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Our flow hash table uses linear probing in which we step backwards through
clusters of adjacent hash entries when we have near collisions.  Usually
that's implemented by flow_hash_probe().  However, due to some details we
need a second implementation in flowside_lookup().  An embarrassing
oversight in rebasing from earlier versions has mean that version is
incorrect, trying to step forward through clusters rather than backward.

In situations with the right sorts of has near-collisions this can lead to
us not associating an ACK from the tap device with the right flow, leaving
it in a not-quite-established state.  If the remote peer does a shutdown()
at the right time, this can lead to a storm of EPOLLRDHUP events causing
high CPU load.

Fixes: acca4235c4 ("flow, tcp: Generalise TCP hash table to general flow hash table")
Link: https://bugs.passt.top/show_bug.cgi?id=94
Suggested-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
This commit is contained in:
David Gibson 2024-09-06 15:17:05 +10:00 committed by Stefano Brivio
parent 0ea60e5a77
commit 7ad9f9bd2b
1 changed files with 1 additions and 1 deletions
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flow.c
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@ -697,7 +697,7 @@ static flow_sidx_t flowside_lookup(const struct ctx *c, uint8_t proto,
!(FLOW_PROTO(&flow->f) == proto &&
flow->f.pif[sidx.sidei] == pif &&
flowside_eq(&flow->f.side[sidx.sidei], side)))
b = (b + 1) % FLOW_HASH_SIZE;
b = mod_sub(b, 1, FLOW_HASH_SIZE);
return flow_hashtab[b];
}